Method For Improving The Performance of Nickel Electrodes

ABSTRACT

The invention relates to a method for improving the performance of nickel electrodes in alkali chloride electrolysis by adding water-soluble platinum compounds to the catolyte.

RELATED APPLICATIONS

This application claims benefit to German Patent Application No. 10 2007003 554.5, filed Jan. 24, 2007 which is incorporated by reference in itsentirety for all useful purposes.

1. FIELD OF THE INVENTION

The invention relates to a method for improving the performance ofnickel electrodes in alkali chloride electrolysis.

2. BACKGROUND OF THE INVENTION

In sodium chloride electrolysis, hydrogen is evolved from an alkalinesolution. Conventionally, the cathodes in the process are made of iron,copper, steel, or nickel. Nickel electrodes can be either solid nickelor nickel plated.

As mentioned in Offenlegungsschrift EP 298 055 A1, nickel electrodes canbe coated with a metal from sub-group VIII, especially the platinummetals (inter alia Pt, Ru, Rh, Os, Ir, or Pd), of the periodic system ofthe elements or with an oxide of such a metal or with mixtures thereof.After a calcination process, the corresponding noble metal oxides arethen usually present on the surface.

The electrode so produced can be used, for example, in sodium chlorideelectrolysis as the cathode for hydrogen development. Many coatingvariants are known, because the coating of metal oxides can be modifiedin very different ways so that different compositions form on thesurface of the nickel electrode. According to U.S. Pat. No. 5,035,789,the cathode used is, for example, a ruthenium-oxide-based coating onnickel substrates.

Once in operation, the plating on the nickel electrode degrades andcauses the cell voltage to increase, making necessary to re-coat theelectrode. This is technically complex, because the electrolysis must bestopped and the electrodes must be removed from the electrolytic cells.An object of the invention is, therefore, to find a simpler method forincreasing or restoring performance.

ELTECH has published and offered a technique with which a voltagereduction of from 200 to 300 mV as compared with untreated nickelelectrodes can be achieved. In this technique, a noble-metal-containingsolution of unnamed composition and constituents is applied in situ,i.e. during operation of the electrolysis, to the cathode side of thesodium chloride electrolysis in membrane cells. The solution is to beadded during operation of the cell and is to lower the cell voltage.

According to the teaching of patent specification U.S. Pat. No.4,555,317, iron compounds or finely divided iron is added to thecatolyte in order to lower the cell voltage during sodium chlorideelectrolysis. The ELTECH publication contradicts this teaching, however,because, according to the information from ELTECH, coating the cathodeswith iron is said to interfere with the electrolysis and to increase thecell voltage.

According to the further known Offenlegungsschrift EP 1 487 747 A1, a0.1 to 10 wt. % platinum-containing compound is added to sodium chlorideelectrolysis. The solution of the platinum-containing compound is addedto the water that forms the catolyte, from 0.1 to 2 litres of theaqueous solution of the platinum-compound-containing solution beingadded per litre of water.

According to JP 1011988 A, the activity of a deactivated cathode basedon a Raney nickel structure with low hydrogen overvoltage is restored byadding, into the catolyte, a soluble compound of a metal of the platinumgroup to the sodium hydroxide solution during operation of the sodiumchloride electrolysis. For example, a sodium chloride electrolytic cellwith 32 wt. % sodium hydroxide solution, a salt concentration of 200 g/lof sodium chloride is operated at 90° C. and with a current density of2.35 kA/m². The cathode is subjected to currentless nickelling forpretreatment and then nickel-plated in a nickel bath. Platinum chlorate,for example, was metered into the catolyte as the active compound, whichresulted in a reduction in the cell voltage by 100 mV.

According to U.S. Pat. No. 4,105,516, metal compounds which are to lowerthe hydrogen overvoltage and accordingly reduce the cell voltage areadded to the catolyte during the electrolysis of alkali metal chlorides.The examples given in U.S. Pat. No. 4,105,516 in turn describe themetering and effects that arise by addition of an iron compound added tothe catolyte of a sodium chloride diaphragm laboratory cell. The cellhas an anode, consisting of expanded titanium metal, which is coatedwith ruthenium oxide and titanium oxide. The cathode consists of iron inthe form of extended metal. The examples show the use of cobalt solutionor iron solution at the iron cathode. Reference has already been madeabove to the disadvantages of iron compounds in the treatment of coatednickel electrodes.

According to the further known patent specification U.S. Pat. No.4,555,317, it is known that sodium chloride electrolysis can be startedwith a nickel-coated copper cathode. An initial metering underelectrolysis conditions of the cell was carried out withhexachloroplatinic acid in three steps. In the first step, 2 mg ofplatinum were metered in per 102 cm², i.e. 0.02 mg/cm², in the secondstep about 0.03 mg/cm² and in the third step about 0.2 mg/cm². The cellvoltage was lowered by a total of about 157 mV.

According to U.S. Pat. No. 4,160,704, metal ions having a low hydrogenovervoltage can be added to catolytes of a membrane electrolytic cellfor sodium chloride electrolysis in order to coat the cathode. Theaddition takes place during the electrolysis. However, the only examplegiven is the addition of platinum oxide in order to improve an iron orcopper cathode.

Sodium chloride electrolysis according to the membrane process is knownin the prior art. The process is carried out as follows: asodium-chloride-containing solution is fed to an anode chamber having ananode, and a sodium hydroxide solution is fed to a cathode chamberhaving a cathode. The two chambers are separated by an ion-exchangemembrane. Joining multiple anode and cathode chambers forms anelectrolyser. The product streams from the anode chamber includechlorine and a less concentrated sodium-chloride-containing solution.The product stream from the cathode chamber includes hydrogen, and amore highly concentrated sodium hydroxide solution than was fed thereto.The volume flow of sodium hydroxide solution fed to the cathode chamberis dependent on the current density and the cell design. At a currentdensity of, for example, 4 kA/m² and with the cell design of UHDE,Version BM 3.0, the volume flow of lye to the cathode chamber is, forexample, between from 100 to 300 l/h, with a concentration of the sodiumhydroxide solution that comes off of from 30 to 33 wt. %. Thegeometrically projected cathode area is 2.71 m², this corresponds to themembrane area. The cathode is made of specially coated extended nickelmetal provided with a special coating (manufacturer e.g. DENORA) inorder to lower the hydrogen overvoltage.

The cathode coatings in sodium chloride electrolysis conventionallyconsist of platinum metals, platinum metal oxides or mixtures thereof,such as, for example, a ruthenium/ruthenium oxide mixture. As isdescribed in EP 129 374, the platinum metals that can be used includeruthenium, iridium, platinum, palladium and rhodium. The cathode coatingdoes not have long-term stability, in particular not under conditions inwhich electrolysis does not occur or during interruptions in theelectrolysis, during which pole reversal processes, for example, canoccur. Accordingly, more or less pronounced damage occurs to the coatingover the operating time of the electrolyser. Likewise, impurities whichpass, for example, from the brine into the lye, such as, for example,iron ions, can become deposited on the cathode or especially on theactive centres of the noble-metal-containing coating and as a result candeactivate the coating. The consequence is that the cell voltage rises,with the result that the energy consumption for the production ofchlorine, hydrogen and sodium hydroxide solution increases and theeconomy of the process is markedly impaired.

It is likewise possible for only individual elements to exhibit damageto the cathode coating, and it is not always economical to stop theentire electrolyser therefor and remove the element with the damagedcoating, because this is associated with considerable production lossesand costs.

Methods for improving nickel electrodes for sodium chloride electrolysiswhich are coated with elements of the platinum metals (sub-group VIII ofthe periodic system), referred to hereinbelow as platinum metals, theiroxides or mixtures thereof, have not hitherto been directly known fromthe prior art.

SUMMARY OF THE INVENTION

The object of the invention is, therefore, to develop a specific methodfor improving nickel electrodes coated with platinum metals, platinummetal oxides or mixtures thereof, for use as cathodes in theelectrolysis of sodium chloride, which process can be used whileelectrolysis operation continues and avoids a prolonged interruption inelectrode operation to restore cathode activity.

The invention relates to a method for improving the performance ofnickel electrodes that are used in a membrane sodium chlorideelectrolytic process comprising:

(a) preparing a water-soluble or alkali-soluble platinum solutioncomprising:

-   -   (i) a solvent and    -   (ii) a soluble platinum compound

and

(b) adding the solution to the catolyte.

The invention provides a method for improving the performance of nickelelectrodes having a coating based on platinum metals, platinum metaloxides or mixtures of platinum metals and platinum metal oxides, forsodium chloride electrolysis according to the membrane process,characterised in that, in the electrolysis of sodium chloride, awater-soluble or alkali-soluble platinum compound, in particularhexachloroplatinic acid or especially preferably an alkali platinate,particularly preferably sodium hexachloroplatinate (Na₂PtCl₆) and/orsodium hexahydroxyplatinate (Na₂Pt(OH)₆), is added to the catolyte.

For purposes of the specification, the term “Group VIII metals” includesall metals listed in sub-Group VIII of the Periodic Table, their metaloxides, and any mixtures of the metals and metal oxides.

The term “nickel cathode” includes electrodes used as cathodes that aresolid nickel or nickel plated, regardless of any additional metalcoatings on the electrode.

The term “platinum solution” includes an alkali or water based solutioncontaining at least platinum and the solvent.

DETAILED DESCRIPTION OF THE INVENTION

In this method it is possible in particular either to meter in thesodium hexachloroplatinate in the form of an aqueous solution or inalkaline solution, or the hexachloroplatinic acid is metered directlyinto the catolyte, in particular the sodium hydroxide solution, areaction then taking place with the lye to form the chloroplatinate.

The addition of the platinum compound is effected in particular whilethe electrolysis is taking place, under normal electrolysis conditions,at a current density of from 0.1 to 10 kA/m², particularly preferably ata current density of from 0.5 to 8 kA/m².

In a further preferred form of the platinum addition, the electrolyticvoltage is varied, after the addition of the platinum compound, inparticular in a pulsed manner, in the range from 0 to 5 V in order todeposit platinum in a more finely divided form on the cathode. Thevoltage here describes the voltage between the anode and the cathode.

To that end it can be sufficient, depending on the rectifier used toproduce the electrolytic direct voltage, to lower the cell voltage inorder to use the residual ripple of the rectifier therefor. In analternating voltage in the mentioned voltage range, the residual rippleof the rectifier can result with an amplitude of from 0.5 to 500 mV.Modern rectifiers scarcely possess any residual ripple, but it ispossible to produce a residual ripple artificially. The residual rippleis between 20 and 100 Hz, for example.

If the amplitude is likewise regulated, it can be +100 or −100 mV aroundthe resting potential for the time of the noble metal metering. Theresting potential is the voltage at which no further current flows. Thatpotential is normally about 2.1 to 2.3 V, depending on the celltechnology and membrane used. However, it is also possible in particularto carry out the noble metal metering when the cell voltage is 0 V, inwhich case the amplitude must be chosen greater than the restingpotential.

Higher modulated amplitudes are likewise conceivable.

Platinum metals that can be present in metal or metal oxide form as theelectrode coating on the nickel within the scope of the invention are inparticular ruthenium, iridium, palladium, platinum, rhodium and osmium.

In a further preferred form of the novel method, in addition to theplatinum compound, at least one other further soluble compounds ofsub-group 8 of the periodic system of the elements, in particularcompounds of palladium, iridium, rhodium, osmium or ruthenium, canadditionally be added. Such compounds are used in particular in the formof water-soluble salts or complex acids.

After deactivation has been detected, the addition in the case offirst-time metering is preferably carried out as follows: a platinumcompound is added to the catolyte, in the feed to the cathode chamber,at a cathode area of 2.71 m², from 0.02 to 11 g Pt per cathode element,corresponding to from 0.007 g/m² to 4 g/m², at a current density of from1 to 8 kA/m². The area used as the basis is the geometrically projectedcathode area, which also corresponds to the membrane area. The rate ofmetering can be such that the platinum-containing solution, based on theplatinum content per m² of cathode area, is metered at a rate of from0.001 g Pt/(hm²) to 1 g Pt/(hm²).

The addition can take place at a current density preferably under normaloperating conditions, or alternatively at a higher or lower currentdensity. For example, the addition can take place at a current densityof in particular from 0.1 to 10 kA/m².

The temperature at which the metering of the platinum compoundpreferably takes place is from 70 to 90° C. The metering can also takeplace at a lower temperature, however.

If a further voltage increase is observed when metering is complete,this can immediately be offset by metering again. This metering requiresa markedly smaller amount of noble metal in order to restore theoriginal voltage. Depending on the quality of the brine, the lye or onstoppages, a further, but smaller addition of platinum may be necessarywithin a period of from 1 to 3 weeks. The addition of the platinumcompound to the catolyte can likewise take place in the feed to thecathodes. The required amounts of platinum are to be calculatedaccording to the scale of the damage. In the case of relativelyconsiderable damage, corresponding to a high voltage increase, moreplatinum must be metered in, while correspondingly less platinum must bemetered in in the case of slight damage, corresponding to a slightvoltage increase. Overdosing with platinum does not result in anyfurther improvement or lowering of the cell voltage, however.

The amount, based on the platinum, of the further soluble compounds fromsub-group 8 in the solution to be added is particularly preferably from1 to 50 wt %.

In a preferred embodiment, the variation in the electrolytic voltage canbe effected by superimposing an alternating voltage on the electrolyticvoltage. The frequency of the superimposed alternating voltage is inparticular from 10 to 100 Hz. The amplitude can then be from 10 to 200mV.

By means of the method according to the invention it is possible for thefirst time to effect a voltage reduction by up to 200 mV in the case ofdamaged nickel electrodes coated with ruthenium and/or ruthenium oxidesor mixtures thereof.

The preparation of the alkali platinate can be carried out by reactionof hexachloroplatinic acid with lye. This can be carried out separatelyor directly in situ if, for example, hexachloroplatinic acid is metereddirectly into the sodium hydroxide supply to the elements or to theelectrolyser. The hexachloroplatinic acid is particularly preferablymetered directly into the feed to the elements.

EXAMPLES Example 1

A commercial electrolyser having 144 elements whose nickel cathodes wereprovided with a coating based on ruthenium/ruthenium oxide from Denorawas operated at a mean voltage of 3.12 V. Of these 144 elements, oneexhibited a voltage increased by more than 100 mV as compared with themean value. The following treatment cycle was begun: 65.88 litres of ahexachloroplatinate solution (1.19 g Pt/l) was metered at a rate of10.98 l/h, during operation, into the sodium hydroxide solution (conc.31.5%) of a membrane electrolyser at a current density of 4.18 kA/m²over a period of 6 hours. 78.25 g of platinum thus reached the surfaceof 144 cathodes (surface area of a cathode: 2.71 m²). This correspondsto an amount of platinum of 0.21 g Pt/m². The cell voltage fell onaverage to 3.08 V, the current consumption rose to 4.57 kA/m². Convertedto 4 kA/m², this corresponds to a reduction in the voltage by 80 mV,accordingly from 3.09 to 3.01. Elements having a markedly higher voltagewere no longer present. On the following day, a further 16.44 litres ofthe same solution, corresponding to 0.05 g Pt/m², were metered in. Thecell voltage did not improve further as a result.

After 9 days, the mean voltage rose to 3.02 V (based on 4 kA/m²), sothat further metering of platinum in the form of hexachloroplatinic acidwas carried out. 4.12 litres of the hexachloroplatinate solution (1.19 gPt/l) were thereby metered in uniformly in the course of 2 hours, sothat 4.9 g of platinum reached the surface of 144 cathodes (0.012 gPt/m²). The electrolysis was continued during the metering, the meanvoltage thereafter was 3.01 V.

The cell voltage at a current density of 4 kA/m² was on average 3.09 Vbefore the metering and 3.01 V after the metering, which corresponds toa voltage reduction of 80 mV.

Example 2

A laboratory electrolytic cell was operated as described in Example 1 ata current density of 4 kA/m² at a cell voltage of 3.05 V with a standardcathode coating from Denora on the nickel cathode. After shutting downthe cell without applying a protective potential, damage to the cathodecoating occurred. A protective potential is conventionally appliedduring a shut-down in order to protect the coating of the cathode fromdamage. After re-starting, the cell voltage was 3.17 V.

A solution of hexachloroplatinate having a platinum content of 1250 mg/lPt was metered into the catolyte while the cell was operating. Aftermetering the solution for 2 hours with a metered amount of 5 ml/h, thevoltage fell to 3.04 V. A total of 12.5 mg of platinum (12.5 mg/100 cm²)was added.

Example 3

The test of Example 2 was repeated, but a solution having a platinumconcentration of 250 mg/l was metered in (same metering time and samefeed capacity). Addition here 2.5 mg Pt/100 cm². The voltage fell from3.16 V to 3.07 V, i.e. by 90 mV.

Further additional metering did not bring about any further voltagereduction.

Example 4 Comparison

A laboratory electrolytic cell was operated as described in Example 1 ata current density of 4 kA/m² at a cell voltage of 3.08 V with a standardcathode coating from Denora on nickel electrodes. After shutting downthe cell without applying a protective potential, damage to the cathodecoating occurred. A protective potential is conventionally appliedduring a shut-down in order to protect the coating of the cathode fromdamage. After re-starting, the cell voltage was 3.21 V.

A solution of rhodium(III) chloride having a rhodium content of 125 mg/lwas metered in over a period of 4 hours at 5 ml/h. Metering was thencontinued for a further 2 hours with a solution having a concentrationof 1250 mg/l and at 5 ml/h, as a result of which a further 50 mV voltagereduction was achieved. The voltage reduction was only 60 mV.

All the references described above are incorporated by reference in itsentirety for all useful purposes.

While there is shown and described certain specific structures embodyingthe invention, it will be manifest to those skilled in the art thatvarious modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and that the same is not limited to the particular forms hereinshown and described.

1. A method for improving the performance of nickel electrodes that areused in a membrane sodium chloride electrolytic process comprising: (a)preparing a water-soluble or alkali-soluble platinum solutioncomprising: (i) a solvent and (ii) a soluble platinum compound and (b)adding the solution to the catolyte and thereby forming a coating on thenickel electrode.
 2. The method according to claim 1, wherein theplatinum compound is a water-soluble salt or a complex acid.
 3. Themethod according to claim 1, wherein the platinum solution ishexachloroplatinic acid, or an alkali platinate, or a mixture thereof.4. The method according to claim 1, wherein the soluble platinumcompound is Na₂PtCl₆, or Na₂Pt(OH)₆, or a mixture thereof.
 5. The methodaccording to claim 1, wherein, after the addition of the platinumsolution, the electrolytic voltage is varied in the range from 0V to 5V.6. The method according to claim 4, wherein, after the addition of theplatinum solution, the electrolytic voltage is varied in the range from0V to 5V.
 7. The method according to claim 5, wherein, after theaddition of the platinum solution, the difference in the electrolyticvoltage is from 0.5 to 500 mV.
 8. The method according to claim 6,wherein, after the addition of the platinum solution, the difference inthe electrolytic voltage is from 0.5 to 500 mV.
 9. The method accordingto claim 5, wherein the voltage is varied by pulsing the voltage or bysuperimposing an alternating voltage on the electrolytic voltage, in therange from 0 to 5 V, and by a difference of from 0.5 to 500 mV.
 10. Themethod according to claim 8, wherein the voltage is varied by pulsingthe voltage or by superimposing an alternating voltage on theelectrolytic voltage, in the range from 0 to 5 V, and by a difference offrom 0.5 to 500 mV.
 11. The method according to claim 10, which furthercomprises at least one additional water-soluble compound from Group VIIIof the Periodic Table are added to the platinum solution.
 12. The methodaccording to claim 1, which further comprises at least one additionalwater-soluble compound from selected from the group consisting ofpalladium, iridium, platinum, rhodium, osmium and ruthenium.
 13. Themethod according to claim 11, wherein, the additional water-solublecompound, based on the amount of platinum in the platinum compound, arepresent in a concentration of 1 wt. % to 50 wt. %.
 14. The methodaccording to claim 12, wherein, the additional water-soluble compound,based on the amount of platinum in the platinum compound, are present ina concentration of 1 wt. % to 50 wt. %.
 15. The method according toclaim 1, wherein the water soluble or alkali soluble platinum compoundsolution is metered at a rate of 0.001 g Pt/(h*m²) to 1 g Pt/(h*m²). 16.The method according to claim 14, wherein the water soluble or alkalisoluble platinum compound solution is metered at a rate of 0.001 gPt/(h*m²) to 1 g Pt/(h*m²).
 17. The method according to claim 15,wherein the temperature at which the metering of the platinum solutionis from 70° C. to 90° C.
 18. The method according to claim 16, whereinthe temperature at which the metering of the platinum solution is from70° C. to 90° C.
 19. The method according to claim 15, wherein themetering of the platinum solution occurs during electrolysis and under acurrent density between 0.1 to 10 kA/m².
 20. The method according toclaim 18, wherein the metering of the platinum solution occurs duringelectrolysis and under a current density between 0.1 to 10 kA/m².